Group B streptococci (GBS) specimens were collected prospectively from infants with early-onset disease and infants who were colonized but did not develop GBS disease between 1995 and 1998 at six U.S. academic centers. The isolates were verified as GBS at the NIH, and antibiotic susceptibilities of all isolates (119 invasive and 227 colonizing strains) were determined. All strains were sensitive to penicillin, vancomycin, chloramphenicol, and cefotaxime. The resistance rates were 20.2 percent to erythromycin and 7.2 percent to clindamycin. (These figures are the overall resistance rates of the isolates collected from 1995-1998). Resistance to erythromycin increased from 15 percent in 1995 to 26 percent in 1997. Type V strains were more resistant to erythromycin than were types Ia, Ib, and III. Resistance was highest in strains from California (32 percent to erythromycin and 12 percent to clindamycin) as compared with a low of 8.5 percent and 2.7 percent, respectively, in Florida. Penicillin continues to be the drug of choice for GBS infection. For women that are penicillin-intolerant, however, selection of an alternative antibiotic should be guided by contemporary regional resistance patterns.